Helium is typically compressed using air-conditioning type compressors, in which a significant amount of oil flows through the compression chamber with the helium in order to keep it cool. The purpose of oil in GM type cryogenic refrigerator compressors is both for lubrication and to absorb the heat produced in the process of helium compression. It is extremely important that the helium delivered to the expander be virtually oil free. Bulk oil separators are used to ensure removal of such oil injected during the compression process. The bulk oil separator serves as an oil reservoir for the system that is drawn down as oil is transferred to the adsorber over the life of the compressor system.
The basic principal of operation of a GM cycle refrigerator is described in U.S. Pat. No. 2,906,101 to McMahon, et al. The GM cycle has become the dominant means of producing cryogenic temperatures in small commercial refrigerators primarily because it can utilize mass produced oil-lubricated air-conditioning compressors to build reliable, long life, refrigerators at minimal cost. GM cycle refrigerators operate well at pressures and power inputs within the design limits of air-conditioning compressors, even though helium is substituted for the design refrigerants. Typically, GM refrigerators operate at a high pressure (Ph) of about 2 MPa (300 pounds per square inch absolute) (psia), and a low pressure of about 0.8 MPa (117 psia).
Air-conditioning compressors are built in a wide range of sizes and several different designs. Means of providing additional cooling to adapt these compressors to compressing helium are different for different compressors. For example, compressors that draw approximately 200 to 600 W are typically reciprocating piston types which are cooled by adding air cooled fins to the compressor shell. Between about 800 to 4,500 W, the most common compressor is a rolling piston type with low pressure return gas flowing directly onto the compression chamber. In rolling piston compressors, oil flows into the compression chamber along with the helium and absorbs heat from the helium as it is being compressed. Most of the oil separates from the helium in the compressor shell which is at high pressure. U.S. Pat. No. 6,488,120 to Longsworth describes the cooling of helium, oil, and the compressor shell by wrapping a water cooling tube around the shell, and further wrapping a helium cooling tube and an oil cooling tube over the water tube. Cooled oil is then injected into the return helium line. In effect, the compressor serves as an oil pump. The amount of oil pumped is typically about 2% of the displacement.
The Hitachi Corporation scroll compressors draw between 5 and 9 kW and have return gas flow directly into the scroll. Oil can be injected into the inlet and discharged with the helium into the shell at high pressure. Most of the oil separates from the helium and collects in the bottom of the compressor, similar to the rolling piston compressor described above. Unlike the smaller compressor, for this type of compressor, cooling the shell with a water cooling tube wrapped around it is not effective. Here, heat from the helium and oil is removed by an after-cooler, that is external to the compressor shell, which is either air or water cooled.
The Copeland corporation scroll compressors for air-conditioning service draw between 5 and 15 kW. These compressors differ from the Hitachi design in that return gas flows into the shell, which is at low pressure, rather than directly into the scroll. In the standard vertical orientation, the scroll is above the motor. No means exist to have cooling oil flow into the compression chamber with the helium. Copeland has modified two compressors, a 5 and a 7.5 kW compressor, to circulate oil for cooling helium by collecting high pressure oil in the discharge plenum above the scroll and having it flow out through a special port to be cooled in an external after-cooler. Another special return port brings oil back into the scroll near low pressure where it mixes with helium that is being compressed.
A description of the construction and operation of a scroll compressor, and the specific changes to adapt the Copeland standard unit to compressing helium, is found in U.S. Pat. No. 6,017,205 to Weatherston, et al. A compressor system that uses the larger of the two compressors that are manufactured for helium service, i.e., Model HC-10® compressor (SHI-APD Cryogenics), together with a description of the entire compressor system, of which the compressor is an essential component, is described in R. C. Longsworth, “Helium Compressor for GM and Pulse-tube Expanders”, in “Advances in Cryogenic Engineering”, Vol. 47, Amer. Inst. of Physics, 2002, pp 691-697.
To reduce the cost of applying the above scroll compressors to applications that require oil injection for cooling, Copeland successfully oriented the compressors horizontally. In the horizontal orientation, oil in the bottom of the compressor at low pressure flows into the scroll due to gravity along with the gas being compressed. The only modification to a standard vertical compressor is the addition of a port at the traditional bottom center of the compressor. In the horizontal orientation, oil, which would normally be pumped from the oil sump in the traditional bottom of the compressor up the drive shaft to lubricate the bearings and scroll, is directed at the end of the drive shaft after it is cooled in an after-cooler. The amount of oil that is circulated is much greater than the amount that is needed to lubricate the bearings. Most of the oil bypasses the motor and flows directly into the compressor shell near the inlet to the compression chamber in the scroll set. This not only reduces the input power and noise level, but it also results in near constant oil levels in the compressor. The bulk oil separator, which is external to the compressor, serves as the oil reservoir for the compressor system. Conventional vertical separators such as Model 603® (Temprite), produce a low separation efficiency and are difficult to fit in the available space. Using a scroll compressor oriented horizontally offered space underneath the compressor for a horizontal bulk oil separator.
The “Horizontal Oil Separator/Reservoir” described in U.S. Pat. No. 5,553,460 to P. E. Isaacs has an oil separator section that is separated from an oil reservoir section and is at a slightly higher pressure so that oil is transferred from the bottom of the separator section to the top of the reservoir section where it is above the level of the oil there. Oil flows out of the reservoir section through a tube that picks it up from the bottom of the reservoir. This arrangement prevents oil from flowing back into the separator region.
Use of the 98 cc displacement Copeland scroll type compressor in a horizontal configuration, and in a package the same size as the smaller HC-10® compressor, imposed severe constraints on the size and orientation of the bulk oil separator.
The use of a horizontal bulk oil separator that fit under the compressor enabled the packaging goals to be achieved but placed severe constraints on the design of a compact bulk oil separator with high separation efficiency and high sensitivity of an oil level switch that enables the amount of oil that can be collected in the adsorber to be limited.
What is needed is a horizontal bulk oil separator which is compact, has a high separation efficacy, and which avoids the problems of prior units.